With the advance of technology, the volume of an electronic device is growing smaller and smaller. However, the performance thereof is growing higher and higher. Thus, the heat-dissipation problem and the design of the cooling fans of electronic devices become critical. In addition to the cooling capability, the designers of cooling fans also have to consider the noise caused by the vibration or the wind shear of high-speed rotation because users cannot accept too much noise. Therefore, the balance between the rotation speed and the noise becomes a key point in designing cooling fans.
As to the control modes of the cooling fans of electronic devices, they may be divided into the manual type and the automatic type. Refer to R.O.C. Patent Publication Nos. 527090 and 545624 for the manual type control mode. Refer to R.O.C. Patent Publication No. 420326, and Patent Nos. M241884 and M250225 for the automatic type control mode. Either of the manual type and the automatic type control modes can be schematically shown with FIGS. 1 and 2. For the conventional control modes proposed in patents or used in products, a temperature-detection element 11 (such as a thermistor) is used to continuously detect the ambient temperature inside an electronic device 50 and then acquire the temperature-variation coefficient; next, the driving power for a cooling fan 40 is modified via the intrinsic impedance variation of the temperature-detection element 11 or the impedance variation of the resistor of the shunt circuit; then, different driving powers generate different rotation speeds of the cooling fan 40. The relationship between the temperature and the rotation speed is commonly linear and like that shown in FIG. 1. However, such a control mode has disadvantages. For example, when the cooling fan 40 is installed inside the power supply of a computer, in addition to the heat generated by the power supply itself, the cooling fan 40 usually has to drive out the heat flows generated by other electronic devices 50 inside the computer (such as CPU and hard drives) also. The location of the power supply together with the locations and quantities of other electronic devices 50 will influence the direction and temperature of the air current of the cooling fan 40 and also influence the cooling capability and the noise caused by wind shear. Owing to the linear impedance variation and the impedance attenuation of the temperature-detection element 11, the manufacturers are hard to appropriately arrange the location of the power supply to achieve a well balance between the rotation speed and the noise. Further, the cooling fans 40 are driven by a single input power source 10 in the conventional technology. When the temperatures of the heat flows of other electronic devices 50 of the computer are too high, the temperature-detection element 11 will detect the high-temperature heat flows of other electronic devices 50, and the cooling fan 40 inside the power supply will thus constantly operate at a high speed. Such a phenomenon not only occurs in the cooling fan 40 of the power supply but also occurs in all the cooling fans 40 inside the computer, i.e. after the computer has operated for a period of time, almost all the cooling fans 40 operate at high speed. Such a case generates high noise, which is unwelcome in the market.